Profiled strips extruded of polymeric materials are widely used in automobile body applications as molding pieces. Molding pieces such as body side moldings or wheel well moldings are used to protect a vehicle body from impact damage and to protect edges of sheet metal panels from corrosion. Most of these molding pieces are extruded of thermoplastic materials such as PVC (polyvinyl chloride), thermoplastic rubber, thermoplastic urethane, or other flexible polymeric materials. PVC is one of the most widely used materials for its low cost and good physical properties.
Molding pieces such as body side moldings may be extruded of a plastic material alone or extruded in an extrusion coating process whereby plastic material is extruded onto a metal reinforcing substrate. In an extrusion coating process where plastic material is combined with a rigid metal substrate, a cross-head extrusion technique is used. In cross-head extrusion, a continuous length of metal reinforcing substrate is pulled through an extrusion die at 90.degree. angle to the direction of the plastic flow while molten plastic material is being extruded onto the metal substrate. The metal reinforcing substrate is usually cleaned and dried through a multi-station process and formed by a series of metal roll formers prior to the entry into the extrusion die.
In the extrusion of vinyl/metal substrate reinforced molding pieces, the metal reinforcing substrate slidingly engaging a mandrel in the cross-head die. The mandrel holds the continuous length of the metal reinforcing substrate in place when it passes through the cross-head die such that the thickness of the extrusion coated plastic material on the substrate may be held constant. In the practice of an extrusion coating process, this is difficult to achieve because of the constant wear of the mandrel surface. As the surface of the mandrel starts wearing down, the metal reinforcing substrate riding on the mandrel surface is being pushed away further from the die face. This would gradually lead to a thicker part since more plastic is being extruded onto the surface of the reinforcing substrate.
Traditionally, an extruder operator would have to stop the extrusion coating process and then raising the mandrel surface by using shim plates in order to maintain the constant distance between the die face and the mandrel surface. This is a very slow and time-consuming process since the extrusion process must be shut down and that the shimming process is performed by trial and error. Furthermore, since it is laborious to start an extrusion coating line and tuning the process into specification, any adjustment of the extrusion die that requires the shutting down of the process would not be desirable.
It is, therefore, an object of the present invention to provide an adjustable extrusion coating die that can be adjusted on the flight, i.e., during the extrusion run.
It is another object of the present invention to provide an adjustable extrusion coating die that can be easily adjusted mechanically during the extrusion run.
It is a further object of the present invention to provide a method of extrusion coating in which an adjustable extrusion coating die is utilized such that the thickness of the extruder plastic material can be adjusted during the extrusion run without shutting down the process.